1. Field of the Invention
The present invention relates generally to microwave ovens, and more particularly, to an inverter type microwave oven that drives a magnetron of the microwave oven in an inverter manner.
2. Description of the Related Art
Generally, microwave ovens are used for thawing foods, such as refrigerated meat, as well as for boiling foods. For frozen dinners in a frozen state, thawing and boiling of foods must be executed sequentially. Further, the microwave ovens are used for warming previously cooked food, such as milk, to a suitable temperature.
In this way, the microwave ovens are frequently used for cooking (thawing or warming) foods using a relatively low output of a magnetron as well as a high output thereof. In thawing, food may be fully boiled under an excessively high output of the magnetron. Further, milk warmed to a suitable temperature is frequently consumed to help individuals suffering from insomnia. In this case, the boiling of milk must be prevented so as to avoid destroying nutritive substances of the milk. However, if the milk is warmed under an excessively high output of the magnetron, the temperature of milk can be increased to the boiling point. Further, if a cooking time is reduced so as to prevent milk from boiling, milk in a lower portion of a container may not be sufficiently warmed.
Therefore, when a microwave oven is used, the output of the magnetron must be sufficiently reduced according to the use of the microwave. In a conventional inverter type microwave oven, the magnetron cannot be used at an output less than 20% of a maximum output of the magnetron. Due to limits of an oscillation voltage of the magnetron and a filament current, which are 3600V and 8.5 to 12 A, respectively, if a duty ratio of a pulse width modulated signal of an inverter is equal to or less than 20% of the maximum output of the magnetron, a secondary side output voltage of a high voltage transformer, which supplies a high voltage to the magnetron, is not sufficient for emission of thermions to occur in the filament. If thermion emissions do not occur, the magnetron cannot oscillate, thus preventing RF signals from being generated. Accordingly, a magnetron with an output at less than 20% of the maximum output of the magnetron cannot be used for cooking.
FIG. 1 is a view showing a relationship between an operating voltage of a magnetron and a filament current in the conventional inverter-type microwave oven. As shown in FIG. 1, in order for the filament current of the inverter type microwave oven to reach 8.5 A to 12 A, an operating voltage must be equal to or greater than 3600V. Thus, when the operating voltage is equal to or greater than 3600V, the filament current reaches 8.5 A to 12 A, and thermion emissions occur, thus enabling the magnetron to oscillate.
FIG. 2 is a view showing a duty ratio of a pulse width modulated signal of an inverter used in the conventional inverter type microwave oven. As shown in FIG. 2, the duty ratio of the pulse width modulated signal of the inverter must be equal to or greater than at least 20% of the maximum output so as to supply enough operating voltage and filament current to allow the magnetron to oscillate.
As described above, since the duty ratio of the pulse width modulated signal of the inverter controlling the output of the magnetron is limited to a range equal to or greater than 20% of the maximum output, an excessively high output of the magnetron is generated in cooking modes, such as thawing and warming modes, such that optimal cooking conditions cannot be realized.